1. Field of the Invention
The present invention relates to the production by centrifugal casting, also known as spin casting, of a short-circuit cage of a rotor of an electric motor, particularly for a rotary electric motor for hermetic compressors of refrigerators and the like.
2. Background
As is known, a cage of this type is made of electrically conductive material, particularly aluminium, and is formed by an upper annular part and a lower annular part, which are connected by a plurality of circumferentially spaced bars. Typically, a cage of this type is made by casting molten aluminium in a rotating mould in which a pack of magnetic laminations has been positioned in advance. The aluminium fills gaps between the walls of the mould and the pack of laminations, and also fills passages created within the pack, thereby forming the aforesaid annular parts and bars respectively. Thus the bars remain embedded in the pack, to which the rotor shaft is subsequently fastened.
A known apparatus of this kind is described in WO-03/097 274.
Cages produced with this apparatus may have non-uniformity and irregularity in their shape, leading to undesirable variations in starting torque, poorer performance and increased vibration during operation.
This non-uniformity is due to the gases which, when captured during casting, cannot be vented towards the outside and therefore remain trapped in the casting.
The object of the present invention is therefore to overcome the drawbacks of the method used at present, thus making it possible to obtain uniform castings of regular shape.